Don't think diabetes will make training more difficult. If anything, a diabetic child uses the bathroom more so they're more likely to go when you set them on the toilet. As a diabetic kid I didn't have accidents during the day, but would have them at night from high blood sugars. My mom couldn't really prevent the overnight accidents, but would wake me up to use the bathroom before she went to bed and put plastic sheets on my bed until it wasn't a problem.
When my (non-diabetic) son was little we bought potty training books, videos, a potty chair that sang and even a dopey book that advocated getting a doll that pees and having a potty training party. I offered to buy my son great toys or take him out for pizza. None of it worked.
He also didn't respond to peer pressure (kids younger than him at daycare were trained) or to being grossed out by having wet pants.
What worked was keeping it simple. If you can tell when your child is going to the bathroom then hustle him onto the toilet. If you can't tell, then try setting him on the toilet occasionally through the day and make a big deal when he goes. Don't bribe. Don't scold.
Your son also has to be ready to make the change. My son was almost 4 when he finally did, but because he was older it took less than a day. I told him that today was the day and we threw away the diaper changing pad and switched to underwear. He had one accident and that was it.
Some kids are more compliant than others and have to use the bathroom more often. That's great, but others are strong willed and have bladders of steel like my son (he uses the bathroom about every 6 hours!). I've also learned that some parents kind of stretch the truth about their child being trained. I work in our church nursery and a couple of the "potty trained" 3-year-olds have frequent accidents. Your kid isn't officially potty trained unless you can leave the house without bringing your child a change of clothes. =)
Try not to stress about it and learn from my mistakes and keep it simple.